Liquid dispenser systems, such as liquid soap and sanitizer dispensers, provide a user with a predetermined amount of liquid upon actuation of the dispenser. In addition, it is sometimes desirable to dispense the liquid in the form of foam by, for example, injecting air into the liquid to create a foamy mixture of liquid and air bubbles. Some dispensers contain a refill unit comprising a container full of liquid soap or sanitizer in a usable concentration and a pump to pump the liquid. During a dispense cycle, activating the pump causes a dose of the liquid to be dispensed. In a foam dispenser, the refill unit typically includes an air pump that causes air to be mixed with the liquid during a dispense cycle to form a foam that is dispensed.